1. Field of the Invention
This invention relates to overhead hoists or crane Systems that are used to transfer especially sensitive or critical loads such as nuclear waste.
2. Description of the Related Technology
A nuclear reactor operates by facilitating a controlled nuclear chain reaction in a reactor core. Typically, the nuclear reaction is fueled by an isotope of uranium, which is supplied to the reactor core in a plurality of elongated fuel rods, which are typically metallic structures that are packed with uranium pellets. Periodically, the fuel rods must be removed and replaced, and the spent nuclear fuel must be safely moved and then stored to avoid contamination of the environment. This spent nuclear fuel remains highly radioactive and is also capable of generating significant thermal energy.
Spent nuclear fuel is preferably stored in a water filled pool or cask. Immersion in water not only dissipates the thermal energy that is generated, it helps in the attenuation of the radiation that is emitted from the spent nuclear fuel. Accordingly, although dry storage and transportation systems are available, spent nuclear fuel is typically shipped from one location to another in sealed, shielded containers that are typically referred to as casks. In certain facilities, spent nuclear fuel is transferred from a first location to a storage cask by first packing the spent nuclear fuel within a canister, and placing the canister within a temporary transfer cask. The transfer cask is lifted by a crane assembly that, according to federal regulations must be designed so that it is single failure proof, and is positioned immediately above a more permanent storage cask. While the temporary transfer cask and the canister remain suspended above the storage cask by the crane assembly, a combination of human and robotic activity is used to tie down the transfer cask so as to immobilize the transfer cask against potentially destabilizing movement with respect to the storage cask. The crane assembly is then disconnected from the transfer cask, and is subsequently connected to the top of the canister. A gate assembly at the bottom of the transfer cask is opened, and the canister is lowered out of the transfer cask and into the storage cask by the crane assembly until it is completely positioned within the more permanent storage cask. The crane assembly must then be disconnected from the canister and reconnected to the temporary transfer cask. The tie-downs are removed, and the temporary transfer cask is withdrawn.
While the foregoing process has been performed innumerable times safely and in compliance with applicable regulations, it requires a substantial amount of time and skill to perform. In addition, to the extent that human labor is needed to immobilize the transfer cask and to undo the immobilization at the end of the procedure there is a risk that personnel may be subjected to potentially harmful radiation.
A need exists for an improved system and process for transferring spent nuclear fuel from a first location to a storage cask that is more time and material efficient than the conventional process described above, and that furthermore minimizes the probability that humans will be placed in a position where they may be exposed to potentially harmful radiation.
Accordingly, it is an object of the invention to provide an improved system and process for transferring spent nuclear fuel from a first location to a storage cask that is more time and material efficient than the conventional process described above, and that furthermore minimizes the probability that humans will be placed in a position where they may be exposed to potentially harmful radiation.
In order to achieve the above and other objects of the invention, a method of moving a canister of spent nuclear fuel from a first location to a storage cask according to a first aspect of the invention includes steps of positioning a canister of spent nuclear fuel within a transfer cask; engaging the transfer cask with a first lifting mechanism; engaging the canister with a second lifting mechanism moving the transfer cask having the canister positioned therein to the vicinity of a storage cask; lowering the canister with respect to the transfer cask with said second lifting mechanism into the storage cask, and wherein the step of lowering the canister with said second lifting mechanism is performed while said first lifting mechanism remains continuously engaged with the transfer cask.
According to a second aspect of the invention, an apparatus for moving a canister of spent nuclear fuel from a first location to a storage cask preferably includes a first lifting mechanism for engaging and lifting a transfer cask in which a canister of spent nuclear fuel is temporarily positioned, the first lifting mechanism comprising a lower engagement assembly that is substantially without freedom of movement with respect to the transfer cask when the first lifting mechanism is engaged with said transfer cask; a second lifting mechanism, the second lifting mechanism being constructed and arranged to engage an upper portion of the canister of spent nuclear fuel, and wherein the second lifting mechanism is mounted on the lower engagement assembly of the first lifting mechanism, whereby the canister of spent nuclear fuel may be lowered with respect to the transfer cask while the first lifting mechanism remains engaged with the transfer cask.